Abstract:
Thin films have applications in many technological fields, particularly in the area of electronics, where they form the basis of integrated circuits [l]. More recently, the techniques used to manufacture VLSI circuits have been applied to mechanical systems, such as micron scale sensors and actuators [2]. As a result, there is now considerable interest in thin film mechanical behavior (e.g. [3, 41). One thin film characteristic that can be critical to device performance is the large stress that is often "locked into" a film after deposition. These growth stresses, referred to as intrinsic stresses when they are not due to thermal expansion effects, can alter mechanical response of thin film diaphrams [5] and resonanting microstructures [6]. When thin film structural members are freed from the substrate, they can catastrophically buckle from compressive stresses, or may fracture due to tensile stresses [7]. Furthermore, gradients in stress through the film thickness can cause freed devices to curl or warp [8].
This thesis concerns stress in undoped polycrystalline silicon (polysilicon) films, which are used in manufacturing both integrated circuits and micro-transducers [I, 21. The polysilicon films examined here were made using low pressure chemical vapor deposition (LPCVD), the current industry standard deposition method. An experimental study was conducted to determine how the stress state varied with deposition conditions. Considering the following processing variables: deposition temperature, gas pressure, silane (SiH,) flow rate, and deposition time. Experiments were performed to measure the average in-plane film stress, as well as the through-thickness stress gradient.
Thin film intrinsic stress has received a great deal of attention in the literature (e.g. [9, lo]), but the cause of the stresses remains unknown in many cases. An understanding of growth stress phenomena would lead to the ability to optimize the stresses in sign and magnitude for specific applications. For example, a thin film for micro-mechanical applications ideally would have a slight tensile stress and no stress gradient, so structures remain flat when released from the substrate. To determine the origin of the stresses in the polysilicon films, the microstructure was studied using both x-ray diffraction and transmission electron microscopy (TEM). X-ray diffraction provided a quantitative measurement of the bulk film texture, or preferred crystallographic orientation, while EM was used to closely examine the film microstructure and morphology.
Four papers have been published based on this project [ll-141. The present work draws from them liberally, expanding on several thoughts and conclusions, and filling in details. Reference [I 11 discusses the texture in the polysilicon films, while reference [12] is an investigation of correlations between stress and texture. The evolution of texture, structure, and stress in the polysilicon films is discussed in [13], and reference [14] deals strictly with the topic of the growth stresses.
Publication date:
May 31, 1991
Publication type:
Master's Thesis
Citation:
Krulevitch, P. (1991). Growth Stresses in Undoped LPCVD Polycrystalline Silicon Films: Research Project. United States: University of California, Berkeley.